Something like that usually happens when there is some other package on your system that depends on something which is provided by the package you remove. E.g. firefox 'provides' a browser. Some package insists on having a browser available. You remove firefox, so apt looks for an alternative that also provides a browser and finds firefox-esr.That's if you go through the alternatives system. It could also be as simple as a package having a depends line in its control file that specifies a number of browsers (e.g. Depends: firefox | firefox-esr). Same effect. One goes, apt picks the other.

I caught everything with "firefox" in the name. But there are probably a lot more r-dependencies in a standard SolydXK install. Purging the only browser is obviously a bad idea. Problem is, according to a search "sudo apt-get install --reinstall" doesn't purge. And just deleting .mozilla and other methods of resetting the profile would also remove the addons which SolydXK has. I wnated a reset to "factory settings" (i.e. what SolydXK originally came with). Would deleting the profile and "sudo apt-get install --reinstall firefox-solydxk-adjustments" achieve that?

I'd just like to know. The problem turned out to be something else anyway. Sometimes I run around in circles while playing helpdesk. I have to learn to cut to the heart of the matter more quickly.